Number of channels

Background

How many independent channels are needed to
achieve high levels of speech understanding? It is difficult to
answer this question using implant patients because of all the confounding
factors (e.g., number of surviving ganglion cells) that may affect their
performance. For example, if a
patient obtains poor auditory performance using 4 channels of stimulation, we
do not know if it is because of the small number of channels or because
there are not enough surviving ganglion cells near the stimulating
electrodes.
Acoustic simulations were used by Dr. Loizou and Dr. Michael Dorman (from Arizona State University) to unconfound the effect of
surviving ganglion cells, etc., and therefore determine how many independent
channels are needed to achieve high auditory performance, assuming that
that all other factors are held equal.

The acoustic simulations mimic the front-end processing of
the implant processor and represent speech as a sum of sinusoids
with time-varying amplitudes and fixed frequencies. The
amplitudes of the sine-waves are computed by filtering the signal
into L logarithmic
frequency bands (or channels) and detecting the envelopes of the filtered waveforms
by full-wave rectification and low-pass filtering (with 400 Hz cutoff frequency).

Listening demos

(To listen to the simulations, just click on the sentences below in the second column, and wait
a few seconds until a small window pops up.)

Sine-wave simulations

How many channels?

How many independent channels are needed to
achieve high levels of speech understanding? The results, with normal-hearing listeners, are given in Figure 1. As it can be seen, the number of
channels needed to reach asymptotic performance depended on the test
material. For the most difficult test, i.e., vowel
recognition, eight channels were needed, while for the least difficult
test, i.e., sentence recognition, five channels were needed. These
results suggest that high levels of speech understanding could be
obtained with 5-8 independent channels of stimulation, at least for adults.

For children, we found that the answer to the above question is different. Children need more channels than adults to understand speech. Click here to find out more about our studies with normal-hearing children and children with implants.

M. Dorman, P. Loizou and D. Rainey (1997). "Speech intelligibility as a function of the number of channels of stimulation for signal processors using sine-wave and noise-band outputs," Journal of Acoustical Society of America,
102(4), 2403-2411.

Simulating the effect of electrode insertion depth

Background
Electrode arrays are inserted only partially into the cochlea, typically
22-30 mm, depending on the state of the
cochlea. The fact that the electrode array is not fully inserted into
the cochlea creates a frequency
mismatch between the analysis frequency and the stimulating frequency.

Acoustic simulations were used by Dr. Loizou and Dr. Michael Dorman (from Arizona State University)
to
determine the effect of electrode insertion depth on
speech understanding for a 5-channel cochlear prosthesis. Different
insertion depths
were simulated ranging from 22 mm to 25 mm insertion.
Greenwood's frequency-to-place equation was used to
determine the
sinewave output frequencies which simulated different electrode depths.
For example, to simulate the 22 mm insertion into the cochlea with 4 mm
electrode spacing, sinewaves
were generated with output frequencies 831, 1566, 2844, 5056 and 8924
Hz. The corresponding
sinewave amplitudes were computed
using analysis filters with center frequencies 418, 748, 1339, 2396,
and 4287 Hz respectively.